A 79-year-old man was admitted to the hospital because of a 20-lb weight loss, low back pain, and leg weakness. He had a 1-year history of fibrotic myelodysplasia, possibly therapy related, with a highly complex chromosome karyotype. Radiologic evaluation showed extensive destructive bone lesions, retroperitoneal lymphadenopathy, and evidence for thoracic spinal cord compression. Core biopsies of a retroperitoneal lymph node showed groups of large, immature-appearing mononuclear cells which, on Wright-stained touch preparation, appeared similar to dysplastic erythroid precursors noted on recent marrow aspirate smears. Immunohistochemical staining showed negativity of neoplastic cells to an extensive panel of nonhematopoietic and myeloid markers, and positivity for CD117, glycophorin A, and CD71, consistent with a diagnosis of erythroblastic sarcoma. This lesion is a very unusual variant of myeloid sarcoma and has been described only rarely in the medical literature.